arXiv subjects

Ying Ran

Publications and source records attributed to Ying Ran.

58 records · Page 4Linked to original sources

arXiv2006

Continuous quantum phase transitions beyond Landau's paradigm in a large-N spin model

We study a large-N generalization of $J_1$-$J_2$ Heisenberg model on square lattice -- an $Sp(2N)$ spin model. The possible quantum spin liquid phases of the $Sp(2N)$ model are studied using the SU(2) projective construction. We find several spin liquid states at least in large $N$ limit, which include SU(2) $π$-flux state, SU(2) chiral spin state and $Z_2$ spin liquid states. All those spin liquid states have non-trivial quantum orders. We show how projective symmetry group, which characterizes quantum order, protects the stability of even gapless spin liquids. We also study the continuous quantum phase transition from the SU(2) $π$-flux state to the SU(2) chiral spin liquid state, and from the SU(2) $π$-flux state to the $Z_2$ spin liquid state. We show that those phase transitions are beyond the paradigm of Landau symmetry-breaking theory. The first phase transition, although a $Z_2$ symmetry breaking transition, contains critical exponents that are different from those obtained from the Ginzburg-Landau theory of Ising universality class. The second transition does not even involve a change of symmetry and has no symmetry breaking order parameter.

cond-mat.str-el↗

arXiv2006

Projected wavefunction study of Spin-1/2 Heisenberg model on the Kagome lattice

We perform a Gutzwiller projected wavefunction study for the spin-1/2 Heisenberg model on the Kagome lattice to compare energies of several spin-liquid states. The result indicates that a U(1)-Dirac spin-liquid state has the lowest energy. Furthermore, even without variational parameters, the energy turns out to be very close to that found by exact diagonalization. We show that such a U(1)-Dirac state represents a quantum phase whose low energy physics is governed by four flavors of two-component Dirac fermions coupled to a U(1) gauge field. These results are discussed in the context of recent experiments on ZnCu$_3$(OH)$_6$Cl$_2$.

cond-mat.str-el↗

arXiv2006

Dichotomy in underdoped high $T_c$ superconductors and spinon-dopon approach to $t$-$t'$-$t''$-$J$ model

We studied underdoped high $T_c$ superconductors using a spinon-dopon approach (or doped-carrier approach) to $t$-$t'$-$t''$-$J$ model, where spinon carries spin and dopon carries both spin and charge. In this approach, the mixing of spinon and dopon describes superconductivity. We found that a nonuniform mixing in $k$-space is most effective in lowering the $t'$ and $t''$ hopping energy. We showed that at mean-field level, the mixing is proportional to quasiparticle spectral weight $Z_-$. We also found a simple monte-carlo algorithm to calculate $Z_{-}$ from the projected spinon-dopon wavefunction, which confirms the mean-field result. Thus the non-uniform mixing caused by $t'$ and $t"$ explains the different electron spectral weights near the nodal and anti-nodal points ({\it i.e.} the dichotomy) observed in underdoped high $T_c$ superconductors. For hole-doped sample, we found that $Z$ is enhanced in the nodal region and suppressed in the anti-nodal region. For electron doped sample, the same approach leads to a suppressed $Z$ in the nodal region and enhanced in the anti-nodal region, in agreement with experimental observations.

cond-mat.str-el↗

arXiv2003

Energy Level Statistics of the U(5) and O(6) Symmetries in the Interacting Boson Model

We study the energy level statistics of the states in U(5) and O(6) dynamical symmetries of the interacting boson model and the high spin states with backbending in U(5) symmetry. In the calculations, the degeneracy resulting from the additional quantum number is eliminated manually. The calculated results indicate that the finite boson number $N$ effect is prominent. When $N$ has a value close to a realistic one, increasing the interaction strength of subgroup O(5) makes the statistics vary from Poisson-type to GOE-type and further recover to Poisson-type. However, in the case of $N \to \infty$, they all tend to be Poisson-type. The fluctuation property of the energy levels with backbending in high spin states in U(5) symmetry involves a signal of shape phase transition between spherical vibration and axial rotation.

nucl-th↗